The invention relates to scanning probe microscopy and, more particularly, to a multi-axis, force-feedback sensor for use in scanning force-probe microscopy.
Scanning force-probe microscopies, particularly atomic force microscopy (AFM), are widely used as a scanning probe technique in diverse applications ranging from research into fundamental material-science problems through nanoscale fabrication and characterization to advanced metrology at the nanometer scale. However, the atomic force microscope (AFM) uses a deflection sensor to measure sample/probe forces that suffers from an inherent mechanical instability that occurs when the rate of change of the force with respect to the interfacial separation becomes equal to the spring constant of the deflecting member. This instability significantly limits the breadth of applicability of AFM techniques. The instability problem has been addressed by the development of the Interfacial Force Microscope (Joyce, S. A. and Houston, J. E., xe2x80x9cA New Force Sensor Incorporating Force-Feedback Control for Interfacial Force Microscopy,xe2x80x9d 1991, Review of Scientific Instruments 62, 710-715; incorporated by reference herein), which utilizes a self-balancing, force-feedback sensor concept. This concept consists of the use of a deflection sensor incorporating an additional means for imposing a counter force to the deflecting portion of the sensor. The deflection is used to signal the presence of a force and a counter force is applied to bring the deflection back to zero, thus eliminating the instability problem and enhancing the applicability of the scanning force-probe technique. The deflecting member is the common plate of a differential capacitor suspended by torsion bars above two individual capacitor electrodes. The common plate acts as kind of a xe2x80x9cteeter totterxe2x80x9d rotating about the torsion-bar axis. If a tip is mounted on one end of the common plate and force is applied to it, the teeter totter will rotate about the torsion bar axis causing the capacitor gap on one end to increase while the other will decrease. This difference in capacitance is detected by an ac-bridge circuit that gives a signal that is directly proportional to the tip displacement. When this displacement signal is fed to a controller circuit capable of developing voltages on the capacitor pads to electrostatically oppose the tip-applied force, then the tip displacement can be reduced to arbitrarily small values. By this means, the sensor is stabilized against the mechanical instability discussed above. However, the use of the RF bridge scheme for measuring deflection is cumbersome, requires reasonably sophisticated electronics and scales poorly with the sensor size. The common plate in the present design is made from a 100 xcexcm thick Si wafer and it, and the torsion bars, are defined by a 25 xcexcm wide trench etched through the wafer thickness. The common plate is 2.5 mm wide and 5 mm long and the length of the torsion bars is also 2.5 mm. This is very large for a scanning force-probe microscope. The AFM cantilevers are generally about 100 xcexcm long and only 1 or 2 xcexcm thick. For ultimate sensitivity, the small size is important. However, detecting small changes in small capacitors is difficult and the capacitance values scale with the square of the pad dimension. Thus, the physical size of the present sensor is a limiting factor and is near its minimum practical value for RF bridge displacement detection. The sensor can only uniquely detect forces along one axis, is difficult and expensive to assemble and is produced with small yields. The high-frequency bridge technique for detecting displacements requires tedious electronics and is limited in sensitivity by the size of the sensor; that is, the sensitivity scales as the area of the capacitor pads and diminishes rapidly with the sensor dimensions.
Young et al. (U.S. Pat. No., 5,705,814) describe various probes and probe detection systems and their use in microscopes, including atomic force microscopes and scanning tunneling microscopes. Among those described include probes containing piezoelectric materials or other materials which produce changes in electrical properties in response to cantilever bending, such as discussed in Albrecht et al. (U.S. Pat. No. 5,345,815). Albrecht et al. describe a cantilever structure with a piezoresistive resistor embedded in the cantilever arm where deflection of the cantilever produces a change in the resistance. Optical methods can also be used in the probe detection system, such as the system described by Shirai et al. (U.S. Pat. No. 6,229,607), where a ray of light is emitted from a light source and the reflected light is used to detect the flexural deforming in the cantilever of the probe system.